I'd like to shade the fire channel by velocity with red and green vectors that can then be used as a input to a vector blur to create a "heat distortion" effect. Can this be done with the current build?

Such 2D vectors cannot be rendered yet. Using the temperature field with a for heat distortion works great though (see attached movies - tutorial is in the works). This may even be preferable to using the velocity because it renders faster and the results look equally plausible. Yet, rendering average motion vectors for such post-effects may still make it into one of the next updates.